Recently, ceramic materials are being sought which have high strength and toughness and are excellent in wear resistance, chipping resistance, oxidation resistance and heat resistance, and which are used for cutting tools, automobile engine parts such as ceramic valves and the like, and heat engine parts such as gas turbine rotors and the like.
As a ceramic material which is excellent in heat resistance and oxidation resistance, silicon oxynitride (Si.sub.2 N.sub.2 O) is known.
As is seen in Yogyo Kyokai Nenkai Yokoshu 1987 (Preprint of the 1987 Annual Meeting of the Ceramic Society of Japan) 3A25 or 25th Yogyo Kiso Toronkai Koen Yoshi Shu (Preprint of the 25th Ceramic Fundamental Forum) 1A18, silicon oxynitride is generally produced adding a metal oxide powder to Si.sub.3 N.sub.4 and SiO.sub.2 as a sintering aid and sintering the resultant mixture.
Due to the addition of the sintering aid, a liquid phase is produced during sintering, and the reaction Si.sub.3 N.sub.4 +SiO.sub.2 .fwdarw.2Si.sub.2 N.sub.2 O which is extremely difficult to proceed in a solid phase is promoted. The sintering of Si.sub.2 N.sub.2 O which is difficult to be sintered is also promoted by this liquid phase.